1. Field of the Invention
The present invention relates to novel thermotropic aromatic copolyesters comprising recurring structural units derived from para-(para-hydroxyphenoxy)benzoic acid or derivative thereof, an aromatic or cycloaliphatic diacid or derivative thereof, a diphenol or derivative thereof and optionally substituted para-hydroxybenzoic acid or derivative thereof.
The (aromatic or aliphatic) rings of the above monomers can either be substituted or unsubstituted.
The novel copolyesters of this invention possess the property of being thermotropic, namely, they can be converted into anisotropic melts that are easy to shape by spinning or molding.
2. Description of the Prior Art
As indicated in EP-A-0,010,051, the anisotropy of polyesters may be demonstrated by the TOT thermo-optical method described in FR-A-2,270,282. When the copolymer is observed in the molten state in optical systems equipped with crossed polarizers, a transmission of the polarized light and creation of a strong birefringence occur, whereas transmission of the light and the birefringence are zero for isotropic products. As a result, the molten masses possess a particular orientation and a relatively high degree of organization, as do the shaped articles formed therefrom, such as threads, films and molded articles, thus providing them, even in the crude state, with improved properties, such as modulus and toughness, which are not usually possessed by isotropic crude products In particular, these molded articles possess advantageous properties in one direction, comparable to those of glass fiber-reinforced plastics.
Typically, the thermotropic copolymers, while possessing excellent mechanical properties, have, on the other hand, high melting points, in general on the order of 270.degree. to 400.degree. C. These high temperatures necessitate appropriate processing equipment, and require an energy consumption higher than that necessary for the processing of conventional resins. In addition, when the melting points are high, the production of molded shaped articles possessing excellent mechanical properties proves difficult or even impossible.
Aromatic copolyesters, thermotropic or otherwise, have long been known to this art:
U.S. Pat. No. 4,067,852 (Celanese), for example, describes wholly aromatic thermotropic copolyesters which can be melt-processed, comprising recurring structural units derived from p-hydroxybenzoic acid, 2,6-naphthalenedicarboxylic acid and hydroquinone.
U.S. Pat. No. 4,161,470 (Celanese) describes thermotropic aromatic copolyesters prepared from 6-hydroxy-2-naphthoic acid and p-hydroxybenzoic acid.
Aromatic copolyesters comprising recurring structural units derived from p-(p-hydroxyphenoxy)benzoic acid or derivative thereof are also known to this art.
Patent Application J 80-133,423 (Hitachi) describes the preparation of copolyesters from (p-(or o- or m-)acetoxyphenoxy)benzoic acid, terephthalic acid and bisphenol diacetate. These copolyesters are not thermotropic and they are, in addition, soluble in chloroform, chlorobenzene and N-methylpyrrolidone, solvents in which the thermotropic copolyesters are generally insoluble.
Patent Application EP 275,164 (ICI) describes copolyesters prepared from p-hydroxybenzoic acid, hydroquinone, isophthalic acid and p-(p-hydroxyphenoxy)benzoic acid, in which the contents of monomers derived from p-(p-hydroxyphenoxy)benzoic acid is not more than 2.5 mol % of the total amount of monomers
Patent Application J 63-101,416 (Asahi) describes thermotropic copolyesters principally derived from p-hydroxybenzoic acid and p-(p-hydroxyphenyl)benzoic acid or p-(p-hydroxyphenoxy)benzoic acid and, optionally, up to 5 mol % of iso- or terephthalic acid and resorcinol relative to the total amount of monomers.
Irrespective of the mole ratio between the units derived from p-hydroxybenzoic acid and those derived from p-(p-hydroxyphenoxy)benzoic acid, since their melting point is above 345.degree. C., their processing into molded shaped articles thus necessitates a temperature above 370.degree. C., which makes them particularly difficult to use. Furthermore, their mechanical properties are not very good.